1. Field of the Invention
This invention relates to a semiconductor device with current confinement structure, and more particularly, it relates to an improvement of the current confining capability of a semiconductor laser device.
2. Prior Art
A buried hetero (BH) structure plays a vital role in stabilizing the transverse mode of a long wavelength semiconductor laser device to be used for optical communications.
As illustrated in FIG. 2 of the accompanying drawings, a known semiconductor laser device having a BH structure on an n-InP substrate 1 typically comprises an n-InP clad layer 2, a GaInAsP (.lambda.g=1.3 .mu.m) active layer 3, a p-InP clad layer 4 and a cap layer 5 as well as a p-InP layer 6 and an n-InP layer 7 for forming current blocking layers and an an n-electrode 8 and a p-electrode 9.
It is a well-known fact that, when embedded pn junctions are used in the current blocking layers of a semiconductor laser device of the above described type, some leakage current flows through such current blocking layers. Attempts and proposals for elimination of this problem are reported in Papers Nos. 1 and 2 listed below.
The Paper No. 2, for instance, describes the result of simulation, showing how the current confinement characteristics of such a device are improved when a strained superlattice of AlInAs/InP is used for part of the current blocking layer.
Paper No. 1: T. Ohtoshi et al., IEEE J. Quantum Electron., QE-25, 1369 (1989) PA0 Paper No. 2: T. Ohtoshi et al., Electron. Lett. Vol. 27, 12 (1989) PA0 Paper No. 3: G. P. Agrawal et al., Long Wavelength Semiconductor Lasers pp. 94-113, pp. 129-141 (Van Nostrand Reinhold Company, N.Y.) PA0 Paper No. 4: F. L. Schuermeyer et al., Appl. Phys. Lett. 55, 1877 (1989) PA0 Paper No. 5: M. Irikawa et al., Jpn. Appl. Phys. 31 (1992) L1351 PA0 Paper No. 6: K. Iga et al., Electron. Lett. 22, 1008 (1986)
So far, however, problems still remain, if pn junctions are used for embedded current blocking layers of such a device. Some of the problems will be briefly discussed below.
Firstly, the threshold current of a long wavelength semiconductor laser device increases in a high-temperature region, because of temperature dependency of the device.
As described in Paper No. 3 listed below, one of the dominant mechanisms of such a degradation in the temperature characteristics of a long-wavelength region a semiconductor laser is attributed to overflowing hot electrons due to Auger effect.
Therefore, the problem here lies in the leakage current due to hot electrons overflowing from the active layer to the current blocking layer of the device.
FIG. 3 of the accompanying drawings illustrates the relationship between the energy level of such overflowing hot electrons and the barrier height of a strained superlattice as described in the Paper No. 2 as listed above.
If the strained superlattice barrier is made of Al.sub.x In.sub.1-x As, the barrier height or the discontinuity of the conduction band edge reaches its maximum at x=0.62, providing a barrier height of about 510 meV against InP as described in both the Paper No. 2 listed above and Paper No. 4 listed below.
On the other hand, since the energy of a hot electron in a semiconductor laser device having a wave length of 1.3 .mu.m is 1.05 eV at its maximum from the conduction edge of the active layer, the overflow of hot electrons having an energy level of greater than 760 meV from the Fermi level Efn in FIG. 3 cannot be suppressed by such a barrier.
Secondly, a pnpn thyristor accompanies a parasitic element as illustrated in FIG. 2 and a amplified large anode current flows as a leakage current (B--B') when a small current (A--A') flows as a gate current. These phenomena were discussed in Japanese Patent Laid-open Publication No. Hei 5- 13882 and U.S. patent application Ser. No. 07/791,767, allowed on Dec. 15, 1992, now U.S. Pat. No. 5,214,662.
It is, therefore, an object of the present invention to provide a semiconductor device comprising current blocking layers having structural features that can effectively suppress overflows of hot electrons that may take place from the active layer to the current blocking layer of the device.
It is another object of the present invention to provide a semiconductor device comprising current blocking layers having structural features that can effectively reduce the current amplification factor of the transistor constituting a pnpn parasitic thyristor in the device as well as the flow rate of the anode current running through the thyristor.